Quantifying Signal Quality with Scattering Parameters (S-Parameters)
At high frequencies, measuring absolute voltage or current waveforms directly becomes impractical. Instead, hardware engineers use Scattering Parameters (S-parameters) to evaluate how high-frequency electromagnetic waves travel through a network, such as a connector or PCB trace.
S-parameters describe the ratio of transmitted, reflected, and absorbed wave power across a specified frequency range. For a 2-port network—where Port 1 is the transmitter input and Port 2 is the receiver output—the two most fundamental parameters are Insertion Loss (S21) and Return Loss (S11).
Insertion Loss (S21): Measuring Transmitted Energy
Insertion loss quantifies how much signal power is lost as a high-frequency wave passes through the connector from Port 1 to Port 2. Expressed in decibels (dB), it is a negative value representing attenuation, calculated as ten times the base-10 logarithm of output power over input power.
Because S21 measures output power relative to input power, an insertion loss reading of −3 dB means that 50% of the signal power has been lost during transmission, while −6 dB corresponds to a 75% power loss.
Physical Causes of Insertion Loss
- Conductor Loss (Skin Effect): At gigahertz frequencies, current concentrates near the outer skin of metal connector pins, increasing effective AC resistance and dissipating energy as heat.
- Dielectric Absorption: Electric fields passing through the plastic connector housing cause molecular friction within the polymer material, absorbing high-frequency energy.
- Radiation and Leakage: Unshielded connectors allow a fraction of the high-frequency wave to radiate into free space as electromagnetic emissions.
Return Loss (S11): Measuring Reflected Energy
Return loss measures the ratio of signal power reflected back to the source (Port 1) relative to the original incident power. It indicates how well the connector's characteristic impedance matches the system impedance.
In engineering specifications, return loss is often expressed as a positive dB number representing the attenuation of the reflected wave (or a negative S11 value directly from Vector Network Analyzer plots). A return loss of −20 dB indicates that only 1% of the incident signal power is reflected back—a sign of an exceptionally well-matched impedance boundary.
Physical Causes of Return Loss
- Impedance Mismatches: Any change in characteristic impedance along the signal path—such as the transition from a 50Ω PCB microstrip to an uncompensated 65Ω connector contact geometry—causes partial reflection of the electromagnetic wave.
- Pin Stubs and Discontinuities: Unterminated pins, via stubs, and sudden geometry changes reflect high-frequency energy back toward the driver.
How Engineers Use Insertion and Return Loss Plots
Evaluating a connector for high-speed designs requires reading frequency-domain S-parameter plots provided by manufacturers:
- Check the Nyquist Frequency Target: Determine the fundamental Nyquist frequency of your data rate (for example, a 28 Gbps NRZ signal has a fundamental Nyquist frequency of 14 GHz). Verify that insertion loss (S21) remains above your system loss budget (e.g., better than −1.5 dB across the connector interface) at this target frequency.
- Identify Resonant Dips: Scan the insertion loss curve for sharp vertical drop-offs. A sudden deep notch in the S21 curve indicates a localized resonance—usually caused by a physical stub—that will destroy signal integrity if it aligns with your signal's clock or harmonic frequencies.
- Enforce Return Loss Limits: Ensure return loss (S11) stays below −10 dB (or better) up to the fundamental frequency. Poor return loss not only degrades the main signal but also sends reflections back into the transmitter IC, creating secondary reflections that close the receiver eye diagram.